The wavelengths of ultraviolet (UV) radiation are between approximately 100 nm and approximately 400 nm, and, within this range, are divided into three bands: band A (UV-A), including wavelengths between approximately 315 nm and 400 nm; band B (UV-B), including wavelengths between approximately 280 and 315 nm; and band C (UV-C), including wavelengths between approximately 100 and 280 nm. Recently, the growth of applications in which ultraviolet radiation or signals are used has increased the interest in devices designed to detect wavelengths in the ultraviolet range, in particular devices having a high value of the signal-to-noise ratio. Such applications include, for example, detection of biological agents via induced fluorescence, sterilization, monitoring of fires in closed environments (via detection of the presence of flames), non-line-of-sight covert communications, and others.
Currently, photomultiplier tubes (PMT) are widely used for the aforesaid applications. However, known photomultiplier tubes present some disadvantages that render use thereof limited or at times disadvantageous. For example, the quantum efficiency of these devices in the range of the UV wavelengths is low, they have large dimensions, high costs, require high biasing voltages, and are fragile.
In recent years, technological progress in the framework of technologies of growth of material on substrates of various types, and more in general in the framework of machining of semiconductors with wide band-gap, such as, for example, silicon carbide (SIC), has favored the development of UV-radiation detectors of an integrated type, made of semiconductor material. Such detectors include, for example, p-i-n photodiodes, avalanche photodiodes, Schottky-barrier diodes, metal-semiconductor-metal photodetectors, and others.
Semiconductor photodetectors may require, for optimized operation, a low dark current, which can be obtained using high-quality materials. In particular, semiconductors with a wide band-gap (wider than that of silicon) offer a low dark current (indicatively some orders of magnitude lower than that of silicon), ideal for the aforementioned application. Even more in particular, silicon carbide of a 4H type (known as 4H—SIC), with a band-gap value of approximately 3.26 eV, has been used experimentally for detection of ultraviolet radiation at a wavelength of 380 nm and shorter, i.e., below the range of the visible (approximately 400 nm), and has proved to be an excellent candidate for the development of UV detectors. Furthermore, since 4H—SiC is a semiconductor with indirect band-gap, photodetectors of this type are advantageous also at high temperatures of use (see, for example, M. Razeghi and A. Rogalski, “Semiconductor ultraviolet detectors”, J. Appl. Phys., vol. 79, No. 10, pp. 7433-7473, 1996).
To improve the sensitivity at a wavelength in the ultraviolet range, Schottky diodes are usually preferred to p-n or p-i-n junction diodes, since generation of the charge carriers occurs at the surface of the semiconductor where a high “built-in” electrical field is present (on account, as is known, of the space-charge region at the metal-semiconductor junction). Furthermore, Schottky diodes are majority-carrier devices, which typically enable a faster response than does a p-n junction. Furthermore, Schottky diodes require a simpler manufacturing process than do p-i-n junction diodes.
Schottky diodes of a conventional type made of silicon carbide for application in the ultraviolet band foresee a top surface provided with a thin metal layer (Ni, Au, Pt) semitransparent to the wavelengths of interest (typically in the 20-50 nm range), with a high value of the Schottky barrier (typically 1.4-1.8 eV) (see, for example, Yan et al., IEEE J. Quantum Electr. 40, 1315, (2004)). The surface metal layer is generally deposited by the sputtering or PVD (physical vapor deposition) technique. However, the sensitivity of these devices is usually low for wavelengths shorter than 300 nm, on account of the absorption of the ultraviolet radiation by the metal layer. To increase the quantum efficiency of SiC photodetectors, it is known to reduce the thickness of the aforementioned metal layer. This approach, however, can cause a problematic control of the uniformity of thickness of the Schottky barrier (which is extremely thin), with consequent problems of poor thermal and mechanical stability.
Direct exposure of the active area (i.e., of the space-charge region of the metal-semiconductor junction) to the radiation that it is desired to detect can be an alternative to improving the sensitivity of the photodetector for low wavelengths. For example, an embodiment has proven satisfactory of a Schottky diode of a vertical type, based upon 4H silicon carbide (4H—SiC) of an interdigitated type, based upon the pinch-off surface effect (see, for example, Mazzillo et al., IEEE Photon Technol. Lect., 21, 1782, (2009)). Such device has enabled an improvement in the efficiency of detection for low wavelengths. In fact, interdigitated geometry leaves exposed surface portions of active area of the photodetector, enabling a high value of efficiency of absorption also at low wavelengths, when the depth of penetration in the silicon carbide is very great (see, for example, A. Sciuto, F. Roccaforte, S. Di Franco, and V. Raineri, “High responsivity 4H—SiC Schottky UV photodiodes based on the pinch-off surface effect”, Appl. Phys. Lect., Vol. 89, 081111, 2006).
Furthermore, use of a weakly doped epitaxial surface layer (with doping of between approximately 1·1014 ions/cm3 and 5·1014 ions/cm3), can favor reaching of the pinch-off condition (i.e., the condition of maximum photon-detection efficiency) between contiguous depleted regions around the interdigitated contacts, with low reverse voltage (also in the photovoltaic regime). Hence, so far, interdigitated Schottky photodiodes represent in the UV-detection range, a promising approach for ensuring high photon detection in all the ranges of UV wavelengths, with low power consumption (see, for example, M. Mazzillo, G. Condorelli, M. E. Castagna, G. Catania, A. Sciuto, F. Roccaforte and V. Raineri, “Highly efficient low reverse biased 4H—SiC Schottky photodiodes for UV-light detection”, IEEE Photon. Technol. Lect., Vol. 21, No. 23, 2009, pp. 1782-1784).
However, the surface of exposed active area of interdigitated Schottky photodiodes must be protected to prevent damage thereto, for example, caused by the presence of dust, by chemical substances present in the environment where the photodetector is used, by handling of the photodetector by an operator, etc. One or more dielectric layers are typically used (for example, employing a plurality of different materials, with different thicknesses), deposited over the entire surface of the photodetector, and also over the contact metallizations. Such dielectric layers should be appropriately deposited to form a cover to protect the surface of the photodetector that will at the same time provide a high efficiency of operation of the photodetector itself. Such cover should be antireflective, such as to optimize matching of the refractive indices at the interface at the various air-cover and cover-semiconductor interfaces.
It is known to provide antireflective and/or protective structures for photodetectors made of silicon oxide (SiO2) and silicon nitride (Si3N4). In particular, an antireflective silicon-oxide layer can be formed on a silicon-carbide layer by thermal oxidation. Further known methods comprise PECVD (plasma-enhanced chemical vapor deposition) and LPCVD (low-pressure chemical vapor deposition).
Regarding growth via thermal oxidation, the silicon-carbide wafer is set in an oxidation oven, where a gas is introduced as precursor. Using oxygen (O2) as precursor gas, a so-called “dry” oxidation is performed. Such process is relatively slow and can lead to a final layer containing crystallographic defects. By introducing a small amount of hydrochloric acid (HCl) into the oxidation oven, or another gas containing chlorine (for example TCE—trichloroethylene), it is possible to reduce the number of crystallographic defects of the layer grown, but there is the disadvantage of obtaining an oxide layer containing a certain percentage of chlorine. Using, instead, water vapor as precursor gas, an oxidation of a “wet” type is performed. The oxide layer that is obtained via the wet-oxidation process presents a lower quality than the oxide layer that can be obtained by dry oxidation, but the rate of growth is higher. Both of the processes require high temperatures, of between 900 and 1200° C.
The PECVD and LPCVD techniques enable deposition of silicon oxide at lower temperatures, but the oxide layer that is obtained presents a poor structural quality, in particular from a stoichiometric standpoint and the standpoint of density.
Irrespective of the particular technique of deposition or growth adopted, further disadvantages of an antireflective layer of silicon oxide on silicon carbide involve high stresses on the SiC—SiO2 interface (in particular, for thick oxides), which can cause generation of leakage current and, in extreme cases, breakdown of the entire wafer.
The performance of 4H—SiC Schottky diodes has been studied for applications of photodetection in the presence of a layer of SiO2 grown thermally on the SiC surface, with purposes of protection thereof. It has been found that pinch-off is affected in the surface region by (trapped) interface charges at the SiC—SiO2 interface. The undesirable effect caused by the trapped charges is even more evident for low surface-doping levels, required when the device is operated at low reverse voltages. Reference may be made in this connection to A. Sciuto, F. Roccaforte, S. Di Franco, V. Raineri, S. Billotta, and G. Bonanno, “Photocurrent gain in 4H—SiC interdigit Schottky UV detectors with thermally grown oxide layer”, Appl. Phys. Lect., Vol. 90, 223507, 2007.
Further disadvantages due to the presence of the protective SiO2 layer comprise a reduction of the response time of the photodetector, an increase of the dark current, and an increase of the leakage currents.
An antireflective silicon-nitride layer can be formed on a silicon-carbide layer via the LPCVD or PECVD deposition technique. Employing the LPCVD technique, the silicon nitride can be deposited in a uniform and reproducible way. The layer obtained has good properties of cover of the edges and good thermal stability. However, the deposition temperatures are high (700-900° C.). The PECVD technique enables a faster deposition at a lower temperature (approximately 400° C.) with the same advantages.
However, on account of the differences in the crystalline structure between silicon carbide and silicon nitride, and/or on account of defects in the crystallographic structure of the silicon-nitride layer, voltages can be set up in the silicon-nitride layer that may even cause breakdown thereof. Furthermore, further stresses in the silicon-nitride layer are caused by variations of temperature, as highlighted in the literature (see, for example, S. Sze, “VLSI Technology”, McGraw-Hill, 1983).
Silicon nitride (for example, deposited by the PECVD technique) has proven more suitable for use as passivation layer. The process of growth can be regulated so that the silicon-nitride layer grown has reduced stress; however, this does not rule out the possible presence of trapped charges at the SiC—Si3N4 interface (see, for example, K. J. Park and G. N. Parsons, “Bulk and interface charge in low temperature silicon nitride for thin film transistors on plastic substrates”, J. Vac. Sci. Technol., vol. 22, No. 6 pp. 2256-2260, 2004). Furthermore, silicon nitride has a band-gap wider than silicon oxide. This means that silicon nitride can have a very high absorption for wavelengths in the 200-300 nm range.
It is evident that the problems generated by the use of protective layers of silicon nitride or silicon oxide are not limited to photodetector devices with substrates made of silicon carbide, but extend to generic substrates of semiconductor material, such as, for example, silicon, gallium, gallium arsenide, gallium nitride, and others still.
Finally, a protective layer is in any case necessary in view of packaging of the photodetector. Currently, packagings comprise a metal load-bearing structure provided with a ultraviolet-degree quartz window, which enables access of photons to the active area of the photodetector; however, the quartz window has a high cost. Plastic packagings are used to reduce costs, but may not guarantee an optimal protection from impact, vibrations, and other external factors. Furthermore, they may not form an adequate protective layer at high transmissivity in the ultraviolet, especially below 300 nm.